Apparatus for charging the surface of photoelectric layers using corona discharge



June 25, 1968 R. H. EPPING 3,390,266

APPARATUS FOR CHARGING THE SURFACE OF PHOTOELECTRIC LAYERS USING CORONA DISCHARGE Filed Oct. 19, 1965 Fig. I

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\ Photoelectric layer W A l 1 4: I

IN I/ E N TOR REINHOLQ HERMANN EPPING ATTORNEYS United States Patent Oflice 3,390,256 Patented June 25, 1968 APPARATUS FOR CHARGING THE SURFACE OF PHOTOELECTRIC LAYERS USING CORONA DISCHARGE Reinhold Hermann Epping, Konigsteinstrasse, 8 Munich 13, Germany Filed Oct. 19, 1965, Ser. No. 497,720 Claims priority, application Germany, Nov. 5, 1964,

8,077 3 Claims. (Cl. 250-495) ABSTRACT OF THE DISCLOSURE The apparatus of the present invention can be used in the method of charging electrophotographic plates described in U .8. Patent 2,543,051.

it is an object of the present invention to provide an improvement of the apparatus disclosed in US. Patent 2,879,395, the disclosure of which is relied upon by applicant.

According to the prior art electrophotography methods of charging are known in which the phenomenon of corona discharge is used for the production of charged carriers. The source of the corona discharge is a very high electric field intensity at the surface of a conductor, whereby the particles acquire velocities sufiiciently high to cause them to ionize air molecules by collision therewith. The critical voltage at which a corona discharge occurs depends on the radius of curvature of the conductor. With a wire of circular cross-section the electric field intensity at the surface of the wire is inversely proportional to the diameter of the wire. For example, with wires having diameters of 3070 microns, D.C. voltages of 3000-7000 will be sufficient to produce corona discharges that will emit radiation in the longer ultraviolet range and in the blue range of the electromagnetic spectrum.

In known devices for charging the surface of photoelectric layers with the help of a corona discharge, a wire of 30-70 micron diameter is positioned about l-Q cm. from the surface to be charged. For charging the surface of the electrophotographic layer, there is applied between the electrically conductive bottom surface of the layer and the wire a DC voltage which is suflicient to produce a corona discharge, and the layer is then moved transversely of the wire.

As a result of unavoidable irregularities of the diameter of the wire and rough places in the surface of the layer, there will occur local electric intensity peaks which along the length of the wire will produce different degrees of ionization, resulting in a non-uniform charging of the surface of the photoelectric layer. It is known that these irregularities of the surface charge can be diminished by interposiug between'the wire and the layer a screen which is kept at a potential between that of the wire and that of the grounded lower surface of the layer. The maximum charge of the layer will then depend on the potential of the screen. The previously known methods of charging the surface of electrophotographic layers by means of corona discharges require long charging times because the ionization density is only small. It has also been necessary for ionization to use very thin wires which are difficult to mount and which are likely to be broken, espe cially by mechanical disturbances.

This invention has for its object to perform the known method of charging the surface of photoelectric layers of the xerographic kind by means of corona discharges in such a manner as to produce much higher ionization density, a more uniform charge on the layer, and greater mechanical stability.

in the present invention in which the surface of the photoelectric layers is charged with the help of a corona discharge and where an electric conductor whose convex outer surface has only a small radius of curvature is positioned in spaced relation to the surface to be charged and is kept at a suitable voltage to maintain the corona discharge, this problem is solved -by applying to the conductor, in addition to the suitable DC. voltage, also a high A.C. voltage of high frequency.

By the use of a high frequency high voltage instead of a DC. voltage for the production of the corona discharge, it is possible to produce greater density of ionization which is nearly independent of the diameter of the wire conductor, so as to permit the use of much thicker and stronger wires. This makes it easier to mount the wires, and since rough places in the wires will then cause fewer irregularities in the charged surface, it will be possible to produce charged surfaces of greater uniformity. The lesser -D.C. voltage which is maintained between the wire and the bottom of the photoelectric layer is to accelerate the positive or negative ions toward the surface of the electrophotographic layer to be charged.

The electric conductor for producing the corona discharge does not need to be in the form of a wire, but can be an electric conductor carrying spaced apart points with definite radii of curvature.

It is also possible in this invention to use a screen grid to control the density of the charge.

In the present invention a bar of electrically conductive material is positioned at a uniform short distance of about 0.5 mm. from the corona producing electric conductor and is kept at a constant potential, e.g. ground potential, and is screened from the conductor by a resinous insulation layer to prevent electric discharges. Such a bar makes possible the higher electric field intensity, resulting in greater ionization density.

The invention will now be further explained with reference to the drawings wherein:

FIGURE 1 shows, partly in section, a schematic representation of one form of this invention; and

FIGURE 2 shows, partly in section, a schematic representation of another form of this invention.

In the device shown in FIGURE 1, :a wire 1 of about 0.12 mm. diameter for producing a corona discharge is suspended inside a downwardly opening trough. The trough 2 is formed of insulating material, e g. of a synthetic resin such as tetrafluorethylene or polypyromelliti-mide. The wire 1 is connected by a suitable conductor 3 and across a capacitor 4 to the outlet of a high frequency high voltage generator 5 delivering, e.g. 4000-6000 volts with a frequency of e.g. 4000 cycles per second. Parallel to wire 1 and at a short distance therefrom, e.g. at a distance of 0.1-2 mm., a metal bar 6 is positioned and is kept at a constant potential, e.g. ground potential. In the device shown in FIGURE 1, the bar '6 is embedded in the wall .of trough 2 which is opposite its open side. The insulating material between the wire 1 and the bar 6 will prevent electric discharges between the two.

The trough 2 faces toward the photoelectric layer 7 to be charged and which is mounted on an electrically conducting bottom layer 8 which is kept at constant potential, e.g. ground potential. Since in this invention the corona discharge is effected by a high frequency A.C. voltage, the wire 1 will be surrounded by a cloud of positive and negative charge carriers, e.g. ions. In order to drive either the positively or negatively charged particles selectively from this cloud upon the photoelectric layer 7, there is provided between the wire 1 and the electrically conducting bottom layer 8 of the photoelectric layer 7, a D.C. voltage of a few hundred volts, depending on the distance of wire 1 from the photoelectric surface 7. The D.C. voltage is supplied by a battery 9, connected between the conductor 3 and the ground. In order to avoid voltage discharges from the high frequency generator 5 to the battery 9, a choke coil is provided. It is also possible to omit the capacitor 4 and choke coil 10 if the end of the secondary of the hi h frequency transformer which is not connected to the corona producing wire is connected directly to the constant voltage part.

It is also possible to use a screen grid 11 between the wire 1 and the photoelectric layer 7, the screen grid being kept at a potential between ground potential and the potential of the wire 1. For example, the screen grid 11 can be connected to a tap on the battery 9. With the help of the screen grid 11, it is possible to control the maximum height of charge on the photoelectric layer. With some layers this is very desirable for preventing electric discharges through the layers. At those places where such electric discharges do occur, the photoelectric layer will remain conducting, which is undesirable, because these places will then be unable to take on any toner particles.

The modification shown in FIGURE 2 differs from that of FIGURE 1 only in that the bar 6 consisting of electrically conductive material is not embedded in the trough 2, but is instead positioned in immediate proximity to the wire 1 between the wire and the photoelectric layer 7. The bar 6 is enveloped by a layer of insulating material to prevent discharges between the wire 1 and the bar 6. In the device shown in FIGURE 2, the bar 6 with its covering of insulating material serves as a diaphragm to shield the photoelectric layer 7 from the radiation produced by the corona discharge. Since in this arrangement the surface of the photoelectric layer 7 is shielded from the radiation produced by the corona discharge, there will also be no loss of charge from said surface as would be likely to occur if the radiation from the corona discharge were permitted to reach the photoelectric layer 7.

In the construction shown in FIGURES 1 and 2, the surface of the photoelectric layer 7 can be charged by either moving the trough 2 over the stationary photoelectric layer, or the latter can be moved under said trough while the latter is kept stationary.

A suitable circuit for the production of a high voltage high frequency electric field is shown in the German book entitled Der Transistor-an amplifying element-by Joachin Dosse, published by R. Oldenbou-rg, Munich, 1957, page 170, but other kinds of circuits for producing frequencies of at least 1000 cycles per second can also be used.

It will be understood that this invention is susceptible to modification in order to adapt it to different usages and conditions and, accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

What is claimed is:

1. An apparatus for electrostatically charging the xerographic surface of a photoelectric layer by using a corona discharge, said apparatus comprising a photoelectric surface to be charged, an electric conductor having a convex outer surface with a small radius of curvature, said conductor positioned at a distance from said surface, a high frequency high voltage generator connected to said conductor by a capacitor, '21 source of relatively lower DC. voltage connected to said conductor by a choke coil, a grounded fiat metal bar shielded from said conductor by a layer of resinous insulation and spaced from said conductor by a distance of 0.1 to 2 mm. and a screen grid at still lower DC. voltage positioned between said conductor and said surface.

2. The apparatus of claim 1, wherein said high frequency is 1000 to 100,000 cycles per second and said high voltage is 4000 to 6000 volts A.C. while said relatively lower DC. voltage is of the order of a few hundred volts.

3. The apparatus of claim 2, wherein said conductor is positioned in an insulating trough, said trough opening toward said surface.

References (Zited UNITED STATES PATENTS 2,777,957 1/1957 Walkup 25049.5 2,864,756 l2/1958 -Rothacker 25049.5 X 2,965,755 12/1960 West 25049.5

WILLIAM F. LINDQUIST, Primary Examiner. 

